Fig. 6.

Effect of ISO on WT β₁-AR compartmentalization in ARVMs. (A) In naive no ISO ARVMs, WT β₁-AR are confined to the surface plasmalemmal membrane. In this compartment, the β₁-AR associates with the PDZ protein SAP97 via its type-1 PDZ. SAP97 in turn binds to the AKAP5/PKA (Gardner et al., 2007) complex. (B) Exposing ARVMs to ISO activated the β₁-AR signaling pathway and increased cAMP, which in turn activated PKA bound to the β₁-AR microdomain. The activated catalytic subunits of PKA phosphorylate many proteins including the β₁-AR on Ser³¹², L-type calcium channels (LTCCs) and many others. In conditions associated with chronic ISO, the agonist-activated β₁-AR translocated away from the SAP97/AKAP5/PKA complex on the surface membrane to punctate invaginations that appear to correspond to T-tubules by an, as yet, undetermined mechanism. (C) Our results show that under chronic ISO conditions, the β₁-AR resides exclusively in these intracellular structures that overlap with the T-tubule-specific dye Di-8 ANEPPS. (D) Upon replacing ISO with the β-blocker ALP, internal β₁-AR recycled in a type-1 PDZ- and phospho-Ser³¹²-dependent manner from their intracellular compartment to the surface plasmalemmal membrane to restore the organization depicted in the no ISO condition shown in (A). RyR, Ryanodine receptors.